Of the xylene isomers, paraxylene is of particularly high value since it is useful in the manufacture of terephthalic acid which is an intermediate in the manufacture of synthetic fibers. Equilibrium mixtures of xylene isomers either alone or in further admixture with ethylbenzene, such as obtained by catalytic reforming of naphtha, generally contain only about 22-24 wt % paraxylene. Separation of paraxylene from such mixtures typically requires superfractionation and multistage refrigeration steps, energy intensive adsorption processes and the like. There is therefore a continuing need to provide processes for producing xylenes which are highly selective for para-isomer.
One known method for producing xylenes involves the alkylation of toluene with methanol over a solid acid catalyst, such as described by Yashima et al. in the Journal of Catalysis 16, 273-280 (1970). These workers reported selective production of paraxylene over the approximate temperature range of 200 to 275° C., with the maximum yield of paraxylene in the mixture of xylenes, i.e., about 50% of the xylene product mixture, being observed at 225° C. Higher temperatures were reported to result in an increase in the yield of meta-xylene and a decrease in production of para and ortho-xylenes.
More recently, selectivities to paraxylene in excess of 90 wt % (based on total C8 aromatic product) have been reported by reacting toluene with methanol in the presence of a catalyst comprising a porous crystalline material, preferably a medium-pore zeolite and particularly ZSM-5, having a Diffusion Parameter for 2,2-dimethylbutane of about 0.1-15 sec−1 when measured at a temperature of 120° C. and a 2,2 dimethylbutane pressure of 60 torr (8 kPa). See U.S. Pat. Nos. 6,423,879 and 6,504,072.
However, irrespective of the selectivity to paraxylene, the alkylation of benzene and/or toluene with methanol inevitably results in the production of a variety of oxygenate by-products as a result of side reactions of methanol with itself and the various aromatic species present. Such oxygenate by-products include water, alcohols, ethers, ketones, aldehydes, acid and phenolic impurities and, depending on their boiling point, are either returned to the alkylation reactor in recycle streams or leave the process through one or more product streams. In particular, the paraxylene-rich product stream tends to contain phenolic impurities such as phenol, methyl phenols and dimethyl phenols. As a result, when the paraxylene is recovered from this product stream, generally by crystallization or by adsorption, the paraxylene product and/or the residual paraxylene-depleted xylene fraction typically contains from one to several hundred ppmw of phenolic impurities. (As used herein, ppmw means parts per million, by weight, relative to the entire weight of whatever stream is referred to). These impurities limit the value of the paraxylene-depleted xylene fraction and generally mean that the fraction can only be used as a blending stream for automotive gasoline. Additionally in the case where phenolic impurities contaminate the paraxylene product fraction there is a potential for downstream processes to be impacted resulting in a decreased downstream performance of the paraxylene purification step(s).
According to the present invention, it has now been found that the concentration of phenolic impurities in a xylene stream produced by alkylation of benzene and/or toluene with methanol can be reduced to trace levels, such as below 0.1 ppmw, by one or more washing treatments with an aqueous solution of a base (caustic). The resultant treated xylene stream, if necessary after water washing to remove any phenate-containing solution (solution containing phenolate or phenylate; the salt of a metal containing the phenoxy radical) and ensuring all trace aqueous base solution, trace metals (for example sodium or potassium ions), and water is removed, can, in embodiments, then be recycled to the other xylene processing units and generate additional paraxylene or the treated xylene stream can be used to manufacture other higher value xylene products such as but not limited to paraxylene. In embodiments, a contaminant-free mixed-xylene by-product may be produced and/or sold for further processing or used as a solvent.